Charged-particle energy analyzer and mass spectrometer incorporating it

ABSTRACT

An electrostatic analyzer (1) for dispersing a beam of charged particles (10) according to their energy comprises two groups (2, 3) of spaced-apart linear electrodes (4, 8, 9, 20) respectively disposed above and below the charged particle beam. The potentials of the electrodes (4, 8, 9, 20) in each group progressively increase from one to the next, thereby providing an electrostatic field in a central plane (7) between the groups which is capable of deflecting the charged particles along different curved trajectories (11, 12) according to their energies. Various mass spectrometers incorporating such an analyzer are also disclosed.

This invention relates to a charged-particle energy analyzer suitablefor use in a double focusing mass spectrometer, and to a massspectrometer incorporating such an analyzer.

The most common type of charged-particle energy analyzer incorporated inmass spectrometers is a cylindrical sector electrostatic analyzer. Suchan analyzer provides energy dispersion and first-order focusing alongonly one axis and is therefore well suited to combination with amagnetic sector mass analyzer to make a double focusing (i.e., bothdirection and velocity focusing) mass spectrometer. Unfortunately,cylindrical sector analyzers comprise two curved electrodes which mustbe machined to very close tolerances and are therefore expensive tomanufacture. Further, the use of a cylindrical sector analyzer in a massspectrometer fitted with a multi-channel detector for the simultaneousdetection of more than one mass-to-charge ratio imposes some seriouslimitations on its performance. Firstly, because of the limited spacingbetween the electrodes, the extent of the focal plane is limited, sothat the range of masses that can be simultaneously imaged is alsolimited. Secondly, the focal plane of such a conventional analyzer isnot usually perpendicular to the direction of travel of the ions leavingit, but inclined at a shallow angle. This further limits the maximumextent of the spectrum which can be simultaneously recorded andcomplicates the design of the detector system. Further, because aconventional analyzer comprises only 2 electrodes, the electrostaticanalyzing field is determined entirely by the shape of the electrodes.This means that the homogeneity of the field cannot be varied and thenumber of aberrations (e.g. focal plane tilt and curvature) which can becorrected is very limited. Similarly, although a greater mass range canbe transmitted by use of an analyzer with a wider gap, it is thennecessary to increase the height of the plates to ensure that the fieldin the vicinity of the ion beam is sufficiently uniform, and this oftenresults in a very large and prohibitively expensive analyzer.

Very few analyzers are known which do not rely on the field generatedbetween two accurately shaped electrodes to define the energy dispersingfield. Auxiliary electrodes are used in prior analyzers to compensatefor the effect of fringing fields where the charged-particle beam entersand leaves the analyzer, but these do not define the main analyzingfield. In these analyzers, one or more electrodes are provided at theentrance and exit of the analyzer and are maintained at potentials suchthat the field between the main electrodes is maintained as close aspossible to the ideal field (e.g., a 1/r field in the case of acylindrical sector analyzer). Similar fringing-field correctorelectrodes may be provided around the edges of a parallel-plateanalyzer, (see, for example, Stolterfoht in DE2648466 A1).

Matsuda (Rev. Sci. Instrum. 1961, vol 32(7), pp 850-852) has described avariable focal length cylindrical sector analyzer which comprises a pairof conventional sector electrodes and a pair of planar auxiliaryelectrodes, respectively disposed above and below the sector electrodes(i.e., displaced along the "z" axis). Application of a potentialdifference between these electrodes results in curvature of theequipotential surfaces along the "z" axis so that the analyzer exhibitssome focusing in the "z" direction. A similar concept is disclosed in JP61-161645 A1 (1986). Matsuda also suggests replacing each of the planarauxiliary electrodes with a number of wires disposed in concentriccircular arcs and applying different potentials to each wire in order tocorrect aberrations, but does not give details as to how this might beachieved in practice. In a later paper (Int. J. Mass Spectrom Ion Phys.,1976, vol 22, pp 95-102), Matsuda suggests using the auxiliaryelectrodes in conjunction with shims on the main electrodes to reducethe height of the main electrodes needed to obtain adequate fieldhomogeneity. In all these analyzers however, the field in the analyzeris principally determined by the main sector electrodes.

Zashkvara and Korsunshii (Sov. Phys. Tech. Phys. 1963 vol 7(7) pp614-619) describe an electrostatic energy analyzer which has focusingproperties along both the "y" and "z" axes in which the mainfield-defining electrodes are disposed either side of thecharged-particle beam along the Y-axis and comprise a stack of flatcylindrical sector electrodes insulated from each other. A resistivepotential divider is used to feed each plate electrode with anappropriate potential. In this way an inhomogeneous field along theanalyzer "z" axis can be created and the focusing properties of theanalyzer adjusted in a similar way to the Matsuda analyzer. TheZashkvara analyzer does not incorporate any electrodes displaced fromthe charged-particle beam along the "z" axis.

Dymovich and Sysoev describe (Phys. Electronics, Moscow, 1965, vol 2, pp15-26 and 27-32) an electrostatic analyzer which is very similar to thatproposed by Matsuda. This analyzer comprises two groups of circular arcelectrodes disposed one above and one below the ion beam, and twocircular main electrodes in a conventional location on either side ofthe ion beam. The analyzer, intended for use in a crossed-field massspectrometer, is described in considerable detail. Second and higherorder aberrations are corrected by adjusting the potential gradientacross the series of auxiliary electrodes in a similar way to thatsuggested by Matsuda. The analyzer as described involved no less than 76circular arc electrodes (of different radii) and does not seem to havebeen adopted in any practical instrument, presumably due to thedifficulty of its manufacture. A complete crossed-field massspectrometer incorporating this electrode structure (called a"multi-electrode electrostatic focusing system, or EFS" by itsdesigners) is described in a later paper (Dymovich, Dorofeev, and Petrov(Phys. Electronics, Moscow, 1966, vol 3, pp 66-75), but according toSoviet Inventors Certificate 851547 (1981) this instrument was found tobe somewhat impractical due to the large size of the electrodestructure. The solution proposed in SU 851547 is to form the circulararc electrodes as metallic deposits on a resistive substrate which iseasier to manufacture, but removes one of the advantages proposed forthe EFS in that the potential gradient between the electrodes isdetermined by the resistive substrate and cannot easily be adjusted tocorrect higher order aberrations.

It is an object of the present invention to provide an improved analyzersuitable for use in a double-focusing mass spectrometer which is easyand cheap to construct.

It is another object of the invention to provide various types of massspectrometers incorporating such an analyzer, and in particular toprovide double-focusing mass spectrometers incorporating such ananalyzer.

Viewed from one aspect, the invention provides an electrostatic analyzerfor dispersing a beam of charged particles according to their energy,said analyzer comprising two groups of spaced-apart linear electrodesrespectively disposed above and below said beam, the more centralelectrodes are disposed, the potential of one electrode of the pairbeing more positive and the potential of the other electrode of the pairbeing more negative than the potential at which ions comprised in saidbeam enter the analyzer and the potentials of all the electrodescomprising each said group progressively increasing from one electrodeto the next, thereby providing in a central plane between said groups ofelectrodes an electrostatic field which is capable of deflecting saidcharged particles along different curved trajectories according to theirenergies.

Preferably the linear electrodes comprising each group are disposedsubstantially parallel to one another and are arrayed in a planeparallel to the central plane of the analyzer.

Further preferably the upper and lower groups of electrodes aresubstantially identical and electrodes in corresponding positions ineach group are maintained at the same potential.

Conveniently, one central electrode of each group is maintained at apotential V and the potentials of the other electrodes in the group aregiven by the polynomial expression:

    V.sub.E =V.sub.M +V.sub.A y.sub.E +V.sub.B y.sub.E.sup.2 +V.sub.C y.sub.E.sup.3 +V.sub.D y.sub.E.sup.4 +. . .               1

in which V_(E) is the potential of a particular electrode, y_(E) is thedistance of said particular electrode from the electrode maintained atV_(M) (positive in one direction, negative in the other), and V_(A),V_(B), V_(C) and V_(D) are constants.

Preferably the potential V_(M) is the potential at which the ions enterthe electrostatic analyzer (i.e., the potential of its entrance slit andcentral trajectory). Alternatively a pair of the central electrodesadjacent to one another may be maintained at potentials respectivelypositive and negative with respect to the potential at which the ionsenter the analyzer.

The field E at any point in the central plane of the analyzer istherefore given by the polynomial expression:

    E=E.sub.O +E.sub.1 y.sub.E +E.sub.2 y.sub.E.sup.2 +E.sub.3 y.sub.E.sup.3 +. . .                                                       2

In equation [2], E_(O) -E₃ are constants and y_(E) is the distance fromthe electrode maintained at potential V_(M) measured in the centralplane. It will be seen that the field generated by an analyzer accordingto the invention is essentially a linear field modified by higher orderterms such as E₂ y_(E) ² and E₃ y_(E) ³ which can be varied byadjustment of the potentials applied to the electrodes. Such a field isunlike that of the prior multi-electrode analyzers which are based oncurved electrodes of circular form and therefore generate a fieldproportional to 1/r (where r is the radius of a particular electrode).

Preferably the coefficient V_(A) (equation [1]) is selected to adjustthe deflection angle of the analyzer, the coefficient V_(B) is selectedto adjust the focal length. If higher order corrections are necessary,the coefficient V_(C) can be selected to set the second order terms(e.g., the angle of the focal plane to the direction of travel of thecharged particles as they leave the analyzer) and the coefficient V_(D)selected to adjust the third order terms, (e.g., the curvature of thefocal plane). Obviously, fourth and even higher order terms can be addedto equation [1] and adjusted if desired.

In principle, an analyzer according to the invention does not requireany electrodes at the side of the ion beam, as in prior conventionalanalyzers, because the field in the vicinity of the charged-particlebeam is defined solely by the groups of electrodes. In practice however,the electrodes at each end of the groups may comprise a single electrodewhich extends through the central plane from the upper group to thelower group, thereby providing fringing field correction at the sides ofthe analyzer.

It will be appreciated that an analyzer according to the invention hasin general a more extensive focal plane than a conventional twoelectrode analyzer of a similar size because the side electrodes, ifprovided at all, may be separated by a much greater distance than arethe electrodes of a conventional analyzer. This is possible because thefringing field errors at the top and bottom of the analyzer, dueprimarily to the proximity of the analyzer vacuum housing, areinsignificant because of the electrode structure, no matter how farapart the main electrodes are spaced, provided that a sufficient numberof electrodes are provided. In this way the need to extend theelectrodes along the "z" axis to reduce these fields is avoided.Further, unlike prior multi-electrode electrostatic analyzers theparallel linear-plate electrode structure allows a compact analyzer tobe constructed very simply.

In a preferred embodiment, entrance and exit fringing field correctionmay be provided by two similar auxiliary electrode assembliesrespectively disposed at the entrance and/or the exit of the mainanalyzer. Conveniently, the upper and lower groups of auxiliaryelectrodes are respectively arrayed in the same planes as the upper andlower groups of electrodes comprised in the main analyzer, and eachauxiliary electrode is disposed in line with a corresponding electrodein the main analyzer.

In a most preferred embodiment, fringing field correctors are providedat the entrance and the exit of the main analyzer and the potential ofthe auxiliary electrodes is the same as the potential of the beam ofcharged particles as it approaches the analyzer. Normally, thispotential is defined by the passage of a beam through a slit maintainedat the same potential as the vacuum housing of the analyzer, usuallyground potential. Conveniently, therefore, the auxiliary electrodes alsomay be maintained at ground potential. The auxiliary electrodeassemblies may conveniently be of identical construction to the mainanalyzer save that the electrodes need only be about 25% of the lengthof the main analyzer electrodes and no insulation is required betweenthem.

An analyzer incorporating fringing field correctors as describedprovides more effective correction than the conventional plate electrodecomprising a slit.

The invention may further provide an electrostatic analyzer whichcomprises two or more multi-electrode segments wherein the electrodesare not all grounded and through which the charged particles passsequentially. For example, such an electrostatic analyzer may be used ina variable dispersion mass spectrometer as described in PCT publicationnumber WO 89/12315. Preferably, fringing field correction is provided oneither side of the segments comprising the main analyzer by means of theauxiliary electrode assemblies described above.

The invention further provides a mass spectrometer comprising a sourceof charged particles, a detector of charged particles, a momentumanalyzer for dispersing a beam of charged particles according to theirmass-to-charge ratios, and an electrostatic analyzer as defined abovefor dispersing a beam of charged particles according to their energy.

Preferably, the momentum analyzer and the energy analyzer will cooperateto form an image on the detector which is both direction and velocityfocused. In this way a double focusing mass spectrometer can be providedmore economically than a conventional spectrometer having a cylindricalsector analyzer. Conveniently, the momentum analyzer is a magneticsector analyzer, which may either precede or succeed the energyanalyzer.

Although the charged-particle detector incorporated in a massspectrometer according to the invention is typically a single-channeldetector such as an electron multiplier or a Faraday cup detector, amulti-channel detector may also advantageously be used, particularlywhen the energy analyzer succeeds the momentum analyzer. In such aspectrometer a greater proportion of the mass spectrum can besimultaneously imaged on the detector than is possible with a sectoranalyzer because in the latter case the narrow gap between the sectorelectrodes imposes a serious limitation on the extent of its focalplane. In a spectrometer according to the invention, this limitation isfar less severe because the gap between the side electrodes can be madevery wide.

When used in a double-focusing spectrometer the coefficients V_(A)-V_(D) (equation [1]) are selected to define the focusingcharacteristics of the analyzer and to minimize aberrations in the finalimage. Thus typically the coefficient V_(A) is selected to define thedeflection angle of the analyzer, the coefficient V_(B) to define thefocal length, and the coefficients V_(C) and V_(D) to define the focalplane tilt (that is, the angle of the focal plane to the direction oftravel of the charged particles as they leave the analyzer) andcurvature. These properties are of course selected in conjunction withthe corresponding properties of the momentum analyzer to provide adouble focusing mass spectrometer. However, it is an easier task toadjust the parameters in an analyzer according to the invention becausethey may be set by simply adjusting electrical potentials rather than bythe geometrical properties of the analyzer such as radius and sectorlength. Consequently, the same analyzer can be employed in differenttypes of mass spectrometer, resulting in considerable cost savings. Itis even possible to alter the first order focusing characteristics,e.g., the position of the ion detector relative to the analyzer (andtherefore the dispersion of the spectrometer) while still maintainingadequate second and higher order focusing. The construction of avariable dispersion (i.e., a "zoom") mass spectrometer is consequentlyfacilitated. Such a spectrometer is particularly useful when amulti-channel detector is employed.

The most convenient way of selecting the electrode potentials in anyanalyzer or spectrometer according to the invention is by the use ofconventional computer ray-tracing programs. These programs allow theposition and shape of the image focal plane to be predicted from a givenset of electrode potentials by repetitively drawing the trajectoriesthrough the analyzer of ions of different energies and startingpositions. An approximate set of potentials can therefore be establishedfor any desired detector position, and final adjustment can be made on acomplete spectrometer if means are provided for adjusting each potentialwithin a narrow range. For example, the electrode potentials may beadjusted for maximum resolution.

The invention will now be described in greater detail by way of exampleonly and by reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an electrostatic analyzer comprisinggroups of linear electrodes;

FIG. 2 is a plot of the potential of the electrodes comprising theanalyzer in an exemplary case;

FIG. 3 is a circuit showing how the potentials may be applied to theelectrodes of the analyzer;

FIG. 4 is a sectional drawing of an analyzer according to the invention;

FIG. 5 is a schematic diagram of a more preferred type of analyzeraccording to the invention;

FIG. 6 is a schematic drawing of one type of mass spectrometer accordingto the invention;

FIG. 7 is a drawing showing an alternative construction of an analyzeraccording to the invention;

FIG. 8 is a schematic drawing of another type of mass spectrometeraccording to the invention; and

FIG. 9 is a schematic drawing of yet another type of mass spectrometeraccording to the invention.

Referring first to FIG. 1, an electrostatic analyzer generally indicatedby 1 comprises two groups 2 and 3 of spaced-apart linear electrodes,(e.g. 4,8,9,20) respectively disposed in planes 5 and 6 which areparallel to the central plane 7 of the analyzer. Potentials are appliedto the electrodes in such a way that they become progressively morepositive from electrodes 8 through to electrodes 9, so that a beam ofpositive charged particles 10, incident as shown and travelling in thecentral plane 7 is deflected within the analyzer in curved trajectories(e.g. 11 and 12) according to the energy of the particles to form agroup of energy dispersed charged-particle beams 13, 14 leaving theanalyzer. In the analyzer shown the two groups 2 and 3 of electrodes aresubstantially identical and electrodes in one group are electricallyconnected to the corresponding electrode in the other group, therebyensuring that there is substantially no field along any axis within theanalyzer perpendicular to planes 5, 6 and 7.

The field within the analyzer is such that an object 15 (defined, forexample, by a narrow slit) located in the analyzer object plane 16 isfocused to a series of energy dispersed images 17, 18 in the analyzerimage plane 19 according to the energy of the charged particlescomprised in the beam 10. For example, charged particles of one energyare deflected along the curved trajectory 11 to form the image 17 andcharged particles of a lower energy are deflected along the curvedtrajectory 12 to form the image 18 at a different place in the imagefocal plane 19. Because there is no field perpendicular to planes 5, 6and 7, the charged particles remain in the same plane in which they aretravelling before they enter the analyzer.

In the analyzer shown in FIG. 1 there are an odd number of electrodes ineach group and the central electrodes 20 of each group are maintained atthe potential V_(M), i.e. the potential of the entrance slit of theanalyzer disposed in the object plane 16 and used to define the object15. Alternatively, a pair of electrodes adjacent to one another in themiddle of the array may be maintained at potentials respectively morepositive and negative with respect to V_(M) may be provided.

The exact shape of the trajectory of ions through the analyzer will ofcourse be dependent on the way in which the potential varies between theelectrodes 4,8, 9 and 20. If the potentials increase linearly fromelectrodes 8 through to electrodes 9, then positive ions will bedeflected as shown in FIG. 1 and the trajectories 11 and 12 will besubstantially parabolic. The field within the analyzer would then besubstantially identical to that which would exist between two parallelstraight electrodes disposed on either side of the ion beam. Asexplained, however, it is more useful to shift the electrode potentialsaccording to the polynomial expression

    V.sub.E =V.sub.M +V.sub.A y.sub.E +V.sub.B y.sub.E.sup.2 +V.sub.C y.sub.E.sup.3 +V.sub.D y.sub.E.sup.4 +. . .               1

where V_(E) is the potential of a particular electrode, V_(M) is thepotential of the central electrode 20, Y_(E) is the distance of thatelectrode from the central electrode, and V_(A), V_(B), V_(C) and V_(D)are constants.

FIG. 2 is a plot of the potential on the electrodes relative to theirposition calculated using the constants V_(A) =1.0, V_(B) =0.2, V_(C)=0.05 and V_(D) =0, which are selected for illustrative purposes only.In FIG. 2, axis 21 represents the potential of the electrode V_(E), andaxis 22 the distance of the electrode from the central electrodes 20(Y_(E)). The graph is drawn with its origin on the central electrode 20(potential V_(M), and y_(E) =0). The broken line 23 represents a linearpotential variation such as would be generated by two conventionallydisposed main electrodes, and the curve 24 indicates the actualpotential variation in an analyzer according to the invention for theconstant values V_(A) =1, V_(B) =0.2, V_(C) =0.05, V_(D) =0. Strictly,curve 24 will comprise a series of short straight lines linking pointslying on the curve where the potential is defined by the electrodeitself. Clearly, it is necessary to use a sufficient number ofelectrodes to ensure that the practical deviations from curve 24 do notsignificantly detract from the analyzer performance. Approximately 11electrodes 4 are sufficient for most applications, but advantage may behad in a very high performance analyzer by using twice that number,resulting in more accurate definition of the field.

It is of course not essential for the electrode defined above as thecentral electrode to be in the physical centre of the array ofelectrodes. It is within the scope of the invention to provide moreelectrodes on one side of the electrical centre than on the other.

FIG. 3 illustrates an electrical circuit used to supply the requiredpotentials to the electrodes 4,8,9 and 20 disposed as in FIG. 1. A powersupply 25 provides equal positive and negative voltages to electrodes 8and 9 as shown, and the central electrodes 20 are connected to the 0volts connection of the supply, which is maintained at potential V_(M),typically ground potential. The other electrodes 4 are fed by taps on apotential divider comprising resistors 26-35 which are selected so thatthe potential on each electrode is as defined by the curve 24 of FIG. 2.Also apparent from FIG. 3 is the connection of each electrode in theupper group 2 to the corresponding electrode in the lower group 3,thereby ensuring that there is substantially no field along an axis(e.g. 36, FIG. 3) perpendicular to planes 5,6, and 7.

If more than one set of electrode potentials is required, two or morechains of resistors may be provided and a multiple switch employed tochange the electrode connections from one chain to the other whenrequired.

In order to provide an easy means of adjusting the electrode potentialsespecially during optimization experiments, each electrode 4 may beconnected to the sliding contact of a potentiometer which forms part ofthe potential divider. Alternatively, the potential of each electrodemay be controlled digitally by means of a conventional voltagecontrolling circuit incorporating a digital-to-analogue converter. Asuitably programmed computer may then be employed to set the electricalpotentials to whatever value is necessary. This method of controllingthe electrode potentials is especially useful when many different setsof electrode potentials are required

Referring next to FIG. 5, an electrostatic analyzer according to theinvention having fringing field correction comprises a main analyzer 65similar to that illustrated in FIG. 1, an entrance fringing fieldcorrector 68, and an exit fringing field corrector 71. The main analyzer65 comprises an upper group of electrodes 66 and a lower group ofelectrodes 67. The electrodes comprised in each group 66 and 67 aremaintained at progressively increasing potentials as previouslydescribed.

The entrance fringing field corrector 68 comprises an upper group ofelectrodes 69 and a lower group of electrodes 70, and the exit fringingfield corrector 71 comprises similar groups 72 and 73. Each of theelectrodes in groups 69, 70, 72 and 73 is aligned with an electrode inthe groups 66 or 67 in order to obtain the best correction, and all theelectrodes in groups 69, 70, 72 and 73 are maintained at the potentialat which the beam enters the analyzer, (typically ground potential). Theside electrodes (e.g., 75, 76, 77) of each group, including those of themain analyzer 65, extend from the upper group (66, 69 or 72) through thecentral plane 74 of the analyzer to form the corresponding sideelectrode of the lower group (67, 70 or 73). These side electrodesprovide fringing field correction at the sides of the analyzer andsignificantly reduce the interference to the electrostatic field insidethe analyzer which might otherwise result from the proximity of agrounded vacuum enclosure. The electrodes in the groups 69, 70, 72 and73 are typically approximately 25% of the lengths of the electrodes inthe groups 66 and 67 which comprise the main analyzer 65.

Referring next to FIG. 4, an electrostatic analyzer suitable for use inthe invention is enclosed in a vacuum housing 37 closed by a lid 38sealed with an `O` ring 39 and secured by bolts 40. A port 41, closed byan `O` ring sealed flange 42 which carries a number of electricalfeedthroughs 43, is provided to allow electrical connection to be madeto the electrodes comprising the analyzer (e.g., lead 44).

The analyzer itself comprises two side electrodes 45, 46 which compriserectangular straight plates which extend through the central plane 7 ofthe analyzer. Side electrodes 45, 46 comprise the end electrodes 8 and 9of the schematically represented electrode structures of FIGS. 1 and 3.As explained, this provides fringing field correction at the edges ofthe analyzer and reduces the distance that the electrode structure needsto extend in order to ensure that the field is properly defined in thevicinity of the ion beam passing through the analyzer.

The side electrodes 45 and 46 are supported on four insulated mountings(two for each electrode) from brackets 47 which are secured to the floorof the vacuum housing 37 with screws 48. Each of the insulated mountingscomprises a ceramic tube 49 and is secured by a screw 50 fitted with aceramic sleeve 51, and a short ceramic tube 52 is fitted under the headof screw 50 as shown.

The upper group 2 and the lower group 3 of electrodes (e.g., 4, 20) areare each supported on two ceramic rods 53 which are located in holesdrilled in the side electrodes 45 and 46. Electrodes 4 are spaced apartby ceramic bushes 54. Each electrode 4 consists of a thin (e.g. 0.5 mm)rectangular metallic plate approximately the same length as the sideelectrodes. The height of the electrodes should be several times (e.g.,five to ten times) their spacing for the effect of fringing fields to benegligible. Typically, the electrodes may be spaced 5 mm apart.

An alternative way in which an analyzer according to the invention canbe constructed is illustrated in FIG. 7. Two insulating (for example,ceramic) plates 78, 79 are spaced apart as shown by metallic sideelectrodes 80, 81 which correspond to the side electrodes 45, 46 shownin FIG. 4. Screws 82 secure the insulating plates 78, 79 to theelectrodes 80 and 81. Each plate 78, 79 comprises a series of ridges 83which are parallel to the side electrodes 80 and 81 and which are coatedwith an electrically conductive deposit 84 (e.g., a metallized film) tocreate the individual electrodes. Electrical connection is made to eachelectrode by means of the connection posts (e.g. 85) which pass throughholes in the plates 78 and 79.

A similar method of construction may also be employed for the entranceand exit fringing field correctors (68, 71, FIG. 5). A complete analyzerincorporating these can be manufactured economically by extending theinsulating plates 78, 79 (FIG. 7) in the direction of the fringing fieldcorrectors and providing ridges similar to the ridges 83 on which theelectrodes comprising the correction assemblies may be deposited.

Although the ridged structure illustrated in FIG. 7 is the mostpreferred form it is possible to form the electrodes simply bydepositing metallic tracks on flat insulating plates. Analyzers soconstructed are not suited to high performance applications, however.

Referring next to FIG. 6, one type of mass spectrometer according to theinvention comprises an ion source 55 which emits a beam of ions 59.These pass in turn through a momentum analyzer, in this case a magneticsector analyzer 56, and a multi-electrode electrostatic analyzer 57, forexample as illustrated in FIG. 4. The mass resolved ion beam 61 whichexits from the analyzer 57 is collected on a charged-particle detector58 which comprises a conventional arrangement of a single channelelectron multiplier and a collector slit which defines the resolution ofthe spectrometer. Alternatively, detector 58 may comprise amulti-channel detector capable of simultaneously recording more than onemass-to-charge ratio.

The magnetic sector analyzer 56 and electrostatic analyzer 57 arepreferably arranged as a double focusing mass spectrometer, i.e., sothat they cooperate to produce an image on the detector which is bothdirection and velocity focused. However it is also within the scope ofthe invention to incorporate a multi-electrode analyzer of the typedescribed in other types of mass spectrometer, for example, as an energyfilter for improving abundance sensitivity in an isotope ratiospectrometer wherein the filter does not cooperate with a momentumanalyzer to form a double focusing mass spectrometer.

Electrical potentials are applied to the electrodes of analyzer 57 bythe power supply 60 which is conveniently similar to that shown in FIG.3. The magnetic sector analyzer 56 is supplied by power supply 62 andthe ion source 55 by power supply 64. A computer 63 is used to controlsupplies 60, 62 and 64. Computer 63 is programmed to set the potentialson the electrodes of analyzer 57 (via the power supply 60) to valueswhich result in an image of the source 55 being formed on the detector58, as in a conventional double focusing mass spectrometer.

The procedure for the design of a double focusing mass spectrometeraccording to the invention is similar to that for the design of aconventional double focusing spectrometer, except that the focusingproperties of the electrostatic analyzer are determined not by itsgeometrical characteristics such as radius and sector length but simplyby the potentials applied to the electrodes. It is therefore a simpletask to change these parameters in order to optimize the performance ofthe completed spectrometer, in contrast to a conventional instrument.

The invention is not limited to a spectrometer wherein the momentumanalyzer precedes the energy analyzer. Advantage is also to be had inthe case where the energy analyzer precedes the momentum analyzer.Similarly, a double focusing spectrometer according to the invention mayor may not involve the formation of an intermediate image at a crossoverbetween the two analyzers, dependent on the type of double focusinggeometry employed.

FIG. 8 is a schematic diagram of an isotope ratio spectrometer accordingto the invention. A charged-particle source 86 generates an ion beam 87comprising ions characteristic of the element(s) in a sample whoseisotopic composition is to be determined. The ion beam 87 enters amulti-electrode analyzer 88 (for example, constructed according to FIG.4) and is deflected and focused to an intermediate energy dispersedimage 89. The ion beam continues through the image 89 into a magneticsector momentum analyzer 90 which disperses the beam into several beams91-93 comprising ions of a different isotope. Beams 91-93 are receivedby a similar number of collectors 94-96, which are typically Faraday cupcollectors for maximum accuracy, so that the isotopic composition of theelement in question can be determined by simultaneous measurement ofsignals generated by collectors 94-96.

It is also possible to reverse the order of the analyzers 88 and 90 sothat the ion beam 87 passes first into the magnetic sector analyzer 90.Because the focal plane of an electrostatic analyzer according to theinvention is more extensive than that of a sector analyzer, it ispossible to receive the mass dispersed ion beam at its entrance and forma series of mass-dispersed energy-focused images with sufficientdispersion to allow the collectors 94-96 to be spaced more widely apartthan would otherwise be possible. This improves the abundancesensitivity of the spectrometer and facilitates the construction of thecollector system.

FIG. 9 illustrates a mass spectrometer according to the invention whichhas three analyzer segments each constructed as described. An ion source97 generates a beam of ions 98 which are dispersed by the magneticsector analyzer 99 into a plurality of beams 100 according to theirmass-to-charge ratios. Beams 100 enter an electrostatic analyzer 106which comprises three segments 101-103 each of which is capable ofdispersing charged particles according to their energy. The analyzer 106cooperates with the analyzer 99 to produce an image on the detector 104which is both direction and velocity focused. Detector 104 is amultichannel detector which is capable of detecting a large number ofdifferent mass-to-charge ratios simultaneously in conjunction with itscontrol and read-out electronics shown schematically at 105. The largenumber of adjustable parameters associated with analyzer 106 allows veryaccurate double focusing to be maintained over a wide range ofdeflection angles and focal lengths of the analyzer 106. Theconstruction of a very high performance multichannel spectrometer withseveral alternative detectors is therefore facilitated.

Analyzer segments 101 and 103 may also be used to change the energy of acharged particle beam as it enters or leaves the segment 102. In thisapplication, the electrodes of segments so used are typically allmaintained at the same potential.

I claim:
 1. An electrostatic analyzer for dispersing a beam of chargedparticles according to their energy, said analyzer comprising an upperand a lower group of spaced apart linear electrodes respectivelydisposed above and below said beam, and means for applying electricalpotentials to said electrodes, each said group comprising a pair ofelectrodes between which one or more central electrodes are disposed,the potential of one electrode of the pair being more positive and thepotential of the other electrode of the pair being more negative thanthe potential at which ions comprised in said beam enter the analyzer,and the potentials of all the electrodes comprising each said groupprogressively increasing from one electrode to the next, therebyproviding in a central plane between said groups of electrodes anelectrostatic field which is capable of deflecting said chargedparticles along different curved trajectories according to theirenergies.
 2. An electrostatic analyzer as claimed in claim 1 wherein thelinear electrodes comprised in each said group are disposedsubstantially parallel to one another and are arrayed in a planeparallel to said central plane.
 3. An electrostatic analyzer as claimedin claim 1 wherein said upper and lower groups are substantiallyidentical and wherein the electrodes in corresponding positions in eachsaid group are maintained at the same potential.
 4. An electrostaticanalyzer as claimed in claim 1 wherein one central electrode of eachgroup is maintained at a potential V_(M) and the potentials of the otherelectrodes in the group are given by the polynomial expression:

    V.sub.E =V.sub.M +V.sub.A y.sub.E +V.sub.B y.sub.E.sup.2 +V.sub.C y.sub.E.sup.3 +V.sub.D y.sub.E.sup.4 +. . .

wherein V_(E) is the potential of a particular electrode, y_(E) is thedistance of said particular electrode from the electrode maintained atV_(M), and V_(A), V_(B), V_(C), and V_(D) are constants.
 5. Anelectrostatic analyzer as claimed in claim 4 which generates anenergy-dispersed image focused at least to the first order and whereinthe coefficients V_(A) and V_(B) are respectively selected to set thedeflection angle and the focal length of the analyzer.
 6. Anelectrostatic analyzer as claimed in claim 5 wherein the coefficientsV_(C) and V_(D) are respectively selected to set the focal plane tiltand the focal plane curvature.
 7. An electrostatic analyzer as claimedin claim 1 wherein the electrodes at each end of said upper group extendthrough said central plane to form the corresponding end electrodes ofsaid lower group in order to provide fringing field correction at thesides of said analyzer.
 8. An electrostatic analyzer as claimed in claim1 wherein said electrodes are electrically conductive members spacedapart by insulators.
 9. An electrostatic analyzer as claimed in claim 1wherein two or more of said electrodes in a said group compriseelectrically conductive material deposited on an insulating plate. 10.An electrostatic analyzer comprising a main analyzer as claimed in claim4 and at least one fringing field corrector disposed adjacent to theentrance (or exit) of said main analyzer, said fringing field correctorcomprising upper and lower groups of spaced-apart auxiliary electrodesdisposed respectively above and below the charged-particle beam as itenters (or leaves) said main analyzer, and wherein all said auxiliaryelectrodes are maintained at the same potential.
 11. An electrostaticanalyzer as claimed in claim 10 wherein said upper and lower groups ofauxiliary electrodes are respectively arrayed in the same planes as saidupper and lower groups of electrodes comprised in said main analyzer,and each said auxiliary electrode is disposed in line with acorresponding electrode in said main analyzer.
 12. An electrostaticanalyzer as claimed in claim 10 wherein fringing field correctors areprovided at the entrance and the exit of said main analyzer and thepotential of said auxiliary electrodes is the same as the potential ofsaid beam of charged particles as it approaches said analyzer.
 13. Anelectrostatic analyzer according to claim 12 wherein said potential ofsaid auxiliary electrodes is ground potential.
 14. An electrostaticanalyzer comprising two or more segments through which the chargedparticles pass sequentially each said segment comprising an analyzer asclaimed in claim
 1. 15. A mass spectrometer comprising a source ofcharged particles, a detector of charged particles, a momentum analyzerfor dispersing a beam of charged particles according to theirmass-to-charge ratio and an electrostatic analyzer as claimed in claim 1for dispersing a beam of charged particles according to their energy.16. A mass spectrometer as claimed in claim 15 wherein said momentumanalyzer and said electrostatic analyzer cooperate to form an image onsaid detector which is both direction and velocity focused.
 17. A massspectrometer as claimed in claim 1 wherein said momentum analyzer is amagnetic sector analyzer.
 18. A mass spectrometer as claimed in claim 16or claim 4 when appended to claim 4 wherein said image is formed in afocal plane, and wherein the coefficient V_(A) and V_(B) are selected tocause at least a part of said focal plane to coincide with saiddetector.
 19. A mass spectrometer as claimed in claim 18 wherein thecoefficients V_(C) and V_(D) are respectively selected to set the focalplane tilt and focal plane curvature to any desired value.
 20. A massspectrometer as claimed in claim 15 wherein said electrostatic analyzeris the final analyzer through which the charged particles pass beforereaching said detector.
 21. A mass spectrometer as claimed in claim 20comprising two or more detectors arrayed in said focal plane forsimultaneously receiving charged particles of different mass-to-chargeratios.